C. pneumoniae is an obligate intracellular bacteria (Christiansen and Birkelund (1992); Grayston et al. (1986)). It has a cell wall structure as Gram negative bacteria with an outer membrane, a periplasmic space, and a cytoplasmic membrane. It is possible to purify the outer membrane from Gram negative bacteria with the detergent sarkosyl. This fraction is named the ‘outer membrane complex (OMC)’ (Caldwell et al. (1981)). The COMC (Chlamydia outer membrane complex) of C. pneumoniae contains four groups of proteins: A high molecular weight protein 98 kDa as determined by SDS-PAGE, a double band of the cysteine rich outer membrane protein 2 (Omp2) protein of 62/60 kDa, the major outer membrane protein (MOMP) of 38 kDa, and the low-molecular weight lipo-protein Omp3 of 12 kDa. The Omp2/Omp3 and MOMP proteins are present in COMC from all Chlamydia species, and these genes have been cloned from both C. trachomatis, C. psittaci and C. pneumoniae. However, the gene encoding 98 kDa protein from C. pneumoniae COMC have not been characterized or cloned.
The current state of C. pneumoniae serology and detection
C. pneumoniae is an obligate intra-cellular bacteria belonging to the genus Chlamydia which can be divided into four species: C. trachomatis, C. pneumoniae, C. psittaci and C.pecorum. Common for the four species is their obligate intra cellular growth, and that they have a biphasic life cycle, with an extracellular infectious particle (the elementary body, EB), and an intercellular replicating form (the reticulate body, RB). In addition the Chlamydia species are characterized by a common lipopolysaccharide (LPS) epitope that is highly immunogenic in human infection. C. trachomatis is causing the human ocular infection (trachoma) and genital infections. C. psittaci is a variable group of animal pathogens where the avian strains can occasionally infect humans and give rise to a severe pneumonia (ornithosis). The first C. pneumoniae isolate was obtained from an eye infection, but it was classified as a non-typable Chlamydia. Under an epidemic outbreak of pneumonia in Finland it was realized that the patients had a positive reaction in the Chlamydia genus specific test, (the lygranum test), and the patients showed a titre increase to the untyped Chlamydia isolates. Similar isolates were obtained in an outbreak of upper respiratory tract infections in Seattle, and the Chlamydia isolates were classified as a new species, Chlamydia pneumoniae (Grayston et al. (1989)). In addition, C. pneumoniae is suggested to be involved in the development of atherosclerotic lesions and for initiating bronchial asthma (Kuo et al. (1995)). These two conditions are thought to be caused by either chronic infections, by a hypersensitivity reaction, or both.
Diagnosis of Chlamydia pneumoniae infections
Diagnosis of acute respiratory tract infection with C. pneumoniae is difficult. Cultivation of C. pneumoniae from patient samples is insensitive, even when proper tissue culture cells are selected for the isolation. A C. pneumoniae specific polymerase chain reaction (PCR) has been developed by Campbell et al.(1992).
Even though Chlamydia pneumoniae has in several studies been detected by this PCR it is debated whether this method is suitable for detection under all clinical situations. The reason for this is, that the cells carrying Chlamydia pneumoniae in acute respiratory infections have not been determined, and that a chronic carrier state is expected but it is unknown in which organs and cells they are present. Furthermore, the PCR test is difficult to perform due to the low yield of these bacteria and due to the presence of inhibitory substances in the patient samples. Therefore, it will be of great value to develop sensitive and specific sero-diagnostics for detecting both acute and chronic infections. Sero-diagnosis of Chlamydia infections is currently based on either genus specific tests as the Lygranum test and ELISA, measuring the antibodies to LPS, or the more species specific tests where antibodies to purified EBs are measured by microimmuno fluorescence (Micro-IF)(Wang et al. (1970)). However, the micro-IF method is read by microscopy, and in order to ensure correct readings the result must be compared to the results with C. trachomatis used as antigen due to the cross-reacting antibodies to the common LPS epitope. Thus, there exists in the art an urgent need for development of reliable methods for species specific diagnosis of Chlamydia pneumoniae, as has been expressed in Kuo et al. (1995); “. . . a rapid reliable laboratory test of infection for the clinical laboratory is a major need in the field”. Furthermore, the possible involvement of C. pneumoniae in atherosclerosis and bronchial asthma clearly warrants the development of an effective vaccine.